With 17% of American adults (36 million) reporting some degree of hearing loss and 14.9% of 6-19 year-old children displaying hearing loss of greater than 16 dB in at least one ear, auditory deficits constitute a significant health concern. These deficits can be genetic, noise-induced, or drug- induced, and often involve hair cells, our auditory detectors and amplifiers. Understanding the mechanisms underlying normal auditory sensation is thus essential to developing new therapeutic interventions for these deficits. This study proposes to elucidate the molecular basis of fast adaptation, one of the processes underlying sound detection by hair bundles, our mechanosensitive organelles. Fast adaptation is a rapid calcium-sensitive negative feedback mechanism that resets the hair bundle's operative range after stimulation. In this study, I propose a novel approach wherein hair bundles are permeabilized to allow direct control of the internal calcium concentration. The calcium concentration will be increased through photolysis of caged calcium compounds, granting me exquisite temporal and spatial control over it, with minimal diffusion-related delays. I will detec a hair bundle's calcium-dependent motion with a flexible glass probe attached to the hair bundle's side. This preparation will allow me to determine whether fast adaptation occurs through one of the several currently proposed theoretical models, and to construct a new model if not. I will then identify a molecular candidate for the mechanism of fast adaptation by studying its properties, such as calcium affinity, and modulating specific candidates known to be present in the mechanotransduction apparatus, such as myosin, calmodulin, and the transmembrane-channel-like proteins (TMC 1 and 2). Elucidating the mechanism of fast adaptation will be a significant development in our understanding of hearing, and will open the door for future investigations of fast adaptation's role in the active process.